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CSIRO PUBLISHING www.publish.csiro.au/journals/fpb Functional Plant Biology, 2009, 36, 960–969 The shoot and root growth of Brachypodium and its potential as a model for wheat and other cereal crops Michelle Watt A,C, Katharina Schneebeli A, Pan Dong A,B and Iain W. Wilson A ACSIRO Plant Industry, GPO Box 1600, Canberra, ACT 2601, Australia. BTriticeae Research Institute, Sichuan Agricultural University, Yaan, Sichuan 625014, China. CCorresponding author. Email: [email protected] Abstract. The grass genetic model Brachypodium (Brachypodium distachyon (L.) Beauv., sequenced line Bd 21) was studied from germination to seed production to assess its potential as a phenotypic model for wheat (Triticum aestivum L.) and other cereal crops. Brachypodium and wheat shoot and root development and anatomy were highly similar. Main stem leaves and tillers (side shoots) emerged at the same time in both grasses in four temperature and light environments. Both developed primary and nodal axile roots at similar leaf stages with the same number and arrangement of vascular xylem tracheary elements (XTEs). Brachypodium, unlike wheat, had an elongated a mesocotyl above the seed and developed only one fine primary axile root from the base of the embryo, while wheat generally has three to five. Roots of both grasses could develop first, second and third order branches that emerged from phloem poles. Both developed up to two nodal axile roots from the coleoptile node at leaf 3, more than eight nodal axile roots from stem nodes after leaf 4, and most (97%) of the deepest roots at flowering were branches. In long days Brachypodium flowered 30 days after emergence, and root systems ceased descent 42 cm from the soil surface, such that mature roots can be studied readily in much smaller soil volumes than wheat. Brachypodium has the overwhelming advantage of a small size, fast life cycle and small genome, and is an excellent model to study cereal root system genetics and function, as well as genes for resource partitioning in whole plants. Additional keywords: Arabidopsis, architecture, genome, monocotyledons, photoperiod, root anatomy, tillering. Introduction growth are similar to Brachypodium, their root–rhizosphere This paper presents a description of Brachypodium distachyon interactions may be similar, as well as root architectures for (L.) Beauv. (Brachypodium) sequenced line Bd 21 shoot and root different moisture and nutrient availabilities. Importantly, at systems from the seedling to grain filling stages. The aim was to maturity Brachypodium is much smaller than cereal crops, assess its potential to identify genes associated with the including other genetic models such as rice (Oryza sativa L.) development and function of wheat (Triticum aestivum L.) and maize (Zea mays L.) and it can be grown in controlled root systems. Brachypodium has been developed as a model conditions in small volumes of soil without greatly crowding for the genetics of cool climate grasses for fuel and crop cereals roots and compromising their growth and function (Fig. 1). (Draper et al. 2001; Opanowicz et al. 2008; Vogel and Bragg Brachypodium presents a new opportunity to identify genes 2009). It has all the desirable features of a genetic model including associated with the growth and function of the roots of a small genome, short generation time between emergence and important cereal crops such as wheat, especially at more seed yield, and easy growth requirements. It is an annual, mature plant stages that are very important in field grown cereals. inbreeding and can be readily transformed (Vogel et al. 2006). Arabidopsis thaliana (L.) is a small-sized genetic model, and The entire genome of diploid line Bd 21 is sequenced and has been the source of most genetic information for roots for publically available (J. Vogel, unpubl. data; http://www. nearly 20 years (Schiefelbein and Benfey 1991; Osmont et al. Brachypodium.org/, accessed 30 September 2009). 2007). However, it is a dicotyledon and member of the Brachypodium originates from the Fertile Crescent, the Brassicaceae family. Dicotyledons develop a tap root system geographic origin of wheat, and is considered a temperate with vascular cambia that produce new xylem and phloem with grass suited to cooler conditions (Vogel et al. 2009). On the development (Esau 1977). Monocotyledons like Brachypodium, phylogeny trees of the grass family Poaceae, the genus and all members of the grass family, develop a ‘fibrous’ root Brachypodium is positioned with the temperate cereal genera system with no vascular cambia, and roots that emerge from the Triticum, Hordeum and Avena in the subfamily Pooideae base of the embryo within the seed (primary or seminal axile (Kellogg 2001; Opanowicz et al. 2008). It can be expected roots), and from successive nodes along the stem (nodal axile that the environmental and soil conditions favourable to wheat roots) (Klepper et al. 1984; Wenzel et al. 1989; Hochholdinger Ó CSIRO 2009 Open Access 10.1071/FP09214 1445-4408/09/110960 Shoot and root growth of Brachypodium Functional Plant Biology 961 root systems traits in crops, especially deep in the soil, is challenging. In the field, this involves direct coring or time- consuming neutron moisture metre readings. By the time of flowering, many crop root systems extend more than 1 m below ground, and it is impossible to capture entire root systems in the field unless constrained in buried tubes. In controlled environments, root systems can be grown to maturity in root boxes greater than 1 m deep with clear faces, to map the spatial distribution of the entire root system to yield (Manschadi et al. 2006). However, these boxes are time- consuming and would be impractical to use on large numbers of lines in a genetics program. Most screening is performed on seedling roots in very small volumes or on agar or other artificial media in the laboratory, and it is unclear if these seedling traits contribute to mature root system traits and yield in the field. Here, we grew Brachypodium alongside wheat in soil in four environments and tracked the shoot and root systems growth and anatomical development, from germination to grain filling, to assess the potential of Brachypodium to identify genes regulating the root systems of wheat (cv. Janz), especially at adult plant stages. Materials and methods Growing environments Brachypodium distachyon (L.) Beauv. (line Bd 21) seeds were kindly provided by Dr D. F. Garvin (USDA-ARS Plant Science Research Unit, University of Minnesota, MN, USA). Brachypodium was grown alongside wheat (Triticum aestivum L. cv. Janz) in the four environments described in Table 1 to compare their timing of root and shoot development, and to establish conditions that resulted in mature Brachypodium root systems that did not touch the bottoms of pots at grain filling. Three of the environments were short day, cooler conditions similar to those that occur in the field during wheat establishment and vegetative growth in Australia and other regions where wheat is grown in cool Fig. 1. Root systems of Triticum aestivum (cv. Janz) (left) and fi winter months; the fourth environment had long days and Brachypodium distachyon (line Bd 21) (right) grown to the ve-leaf stage fl in 50-cm tubes with a mixture of sand and soil in a growth cabinet in cool, warmer temperatures to promote more rapid owering (from short day conditions (environment 3 in Table 1). Bar = 10 cm. Vogel and Bragg 2009). For all experiments, seeds of Brachypodium and wheat were planted with the embryo of the seed facing the base of the pot, et al. 2004; Watt et al. 2008). The water transport through ~2 cm below the soil surface. For studies in the three short day dicotyledons and monocotyledons is different due, in part, to environments, the husks were removed from the Brachypodium their development and anatomy (Bramley et al. 2009). seeds and the seeds planted directly into soil and allowed to Other cereal models such as rice and maize are also very useful germinate in the conditions described above. For the long day for identifying root-associated genes (Hochholdinger et al. 2004; environment studies, the husks were not removed, the seeds Hochholdinger and Zimmermann 2008). However, they grow in planted, soil covered and pots vernalised at 7C in a cold room warm conditions. Rice, in particular, is adapted to waterlogged for 5 days, before transfer to the growth cabinet where the plastic conditions, unlike wheat. Their large adult size means both are cover was removed. Husk removal may have sped up germination difficult to screen in the laboratory to maturity. The root systems but the effect was marginal. Plants were grown in a soil that was a quickly reach the bottom of pots and become crowded and mix of 50% river sand and 50% recycled potting soil (organic impeded, and their deep root development and physiology matter, loam and sand supplemented with lime, 14.4% N, 6.6% P may not be relevant to that of field roots. Mature root system and 5% K). It was chosen because it drains well and washes off architecture and function are valuable to yield because they roots easily and was used to characterise wheat, barley (Hordeum determine the water that can be taken up around flowering and vulgare L.) and triticale (x Triticosecale) root systems in Watt grain filling, which contributes directly to final yield and crop et al. (2008). Plants were grown in PVC tubes of loosely packed water use efficiency (Passioura 1983; Manschadi et al. 2006; soil that were 7 cm wide, either 25 cm or 50 cm deep, and sealed at Kirkegaard et al. 2007). Directly selecting and validating mature the bottom with a Petri dish lid with holes for drainage. All plants 962 Functional Plant Biology M. Watt et al. Table 1. Growing environments for Brachypodium distachyon (line Bd 21) and Triticum aestivum (cv.

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